The present invention relates to a burner using a pulverised fuel.
The recent changes in the price of petroleum products have led to increased interest in burners using a pulverised fuel, in particular coal, and multifuel burners capable of using a solid fuel, a liquid fuel or both at the same time.
The burners commonly used for a liquid or solid fuel comprise two concentric tubes, one of which supplies an air/fuel mixture, this mixture being formed before it is sent to the burner, or alternatively being formed in the burner or at its nozzle, and the other of which supplies pure air and provides it with a rotational movement about the axis. This rotational movement combines with the axial displacement of the gas stream to give a helical movement, which serves to control the divergence of the flame. This air is frequently called "rotational air". The term "pure air" must obviously be understood as meaning any combustion gas not mixed with the fuel, for example oxygen-enriched air.
In some designs, the air/fuel mixture is sent through the peripheral tube of the burner and the rotational air through the central tube, whereas, in other cases, this arrangement is reversed. In either arrangement, an even more central tube, through which additional pure air is sent, is sometimes provided along the axis of the burner.
At constant heat throughput, a flame can be defined by its length and its divergence. It has been found that, for a burner of given geometry, the length of the flame decreases when the speed of the gases ejected by the burner increases, this apparently paradoxical fact resulting from the influence of the turbulences in contact with the secondary air not ejected by the burner. More precisely, it has been found that the length of the flame is approximately inversely proportional to the square root of the axial momentum, that is to say the product of the mass flow of gas and its average axial speed. The divergence of the flame depends on the ratio A/R, A being the gas flow in axial movement (air/fuel mixture plus any additional pure air) and R being the flow of rotational air.
In a coal-fired tubular kiln, for example a cement kiln, it is desirable to operate with a constant flame divergence, and it is necessary for this divergence to be small because of the proximity of the refractory lining and because of the corrosive nature of the coal ash in the molten state. On the other hand, it is desirable to be able to modify the flame length at will, because this factor controls the temperature gradient in the kiln, which is an important parameter in the cement industry because of the physico-chemical phenomena which result therefrom.
With a burner of conventional type, in order to modify the flame length, the axial momentum can only be changed by altering the gas flows, because the speed of the gas streams is fixed by their flow and by the geometry of the burner. If it is desired to modify the flame length without varying the divergence, it is necessary to alter both the flow of axial air and the flow of rotational air simultaneously, in order to keep the ratio A/R at a constant value. This has disadvantages. Two simultaneous adjustments are required and this leads to complications. By altering the flow of air/fuel mixture, it is likely that it will also be necessary to modify the amount of fuel and hence the amount of energy released. In fact, the composition of the air/fuel mixture can only be controlled within relatively narrow limits, because this mixture is not generally formed in the burner itself, and it must carry the fuel into the burner under satisfactory conditions. However, if the total gas flow is varied without modifying the fuel flow, variations in the energy released are nevertheless observed as a result of the following phenomenon: the total amount of air which leaves the burner only represents part of the air used in the flame, the remainder being secondary air taken from the kiln itself. The air sent into the burner is cold, whereas the secondary air is hot; consequently, a variation in the ratio of the flow of the secondary air to the flow of the air delivered by the burner results in an undesirable variation in the flame temperature.
It can be seen that the running of a kiln equipped with a pulverised coal burner is a complicated technique which is difficult to control.